Without limiting the scope of the present invention, its background will be described with reference to downhole testing operations, as an example. Well testing and stimulation operations are commonly conducted on oil and gas wells in order to determine production potential and to enhance the same, if possible. In flow testing a well, a testing string including a tester valve is typically lowered into the well on a string of drill pipe above a packer. After the packer is set, the tester valve is opened and closed periodically to determine formation flow, pressure and rapidity of pressure recovery. Commonly, the operation of such tester valves is responsive to pressure changes in the annulus between the testing string and the wellbore casing. Many such tester valves also provide a biasing source, such as an inert gas like nitrogen, to aid in certain operations of the tester valve, including closure of the tester valve.
In one such arrangement, annulus pressure is used to shift a ball valve assembly in the tester valve from the closed position to the open position. In addition, the annulus pressure is used to charge the biasing source by, for example, compressing nitrogen in a chamber. When the annulus pressure is reduced, the compressed nitrogen is used to shift a ball valve assembly from the open position to the closed position. In this arrangement, a time delay feature, such as a fluid metering section, is used to allow the annulus pressure to first open the ball valve assembly and then charge the nitrogen. For example, it may be desirable to increase the annulus pressure above a certain threshold within one or two minutes in order to open the ball valve assembly, thereafter it may be required that the annulus pressure be maintained at the elevated pressure for another ten or twenty minutes to fully charge the nitrogen.
In certain circumstances, it may be desirable to close the tester valve shortly after opening the tester valve. It has been found, however, that during the period of time delay between opening the ball valve assembly and fully charging the nitrogen, closure of the tester valve is uncertain and in some cases not possible. A need has therefore arisen for an improved tester valve that is operable for flow testing of a well. A need has also arisen for such an improved tester valve that operates responsive to annulus pressure. Further, a need has arisen for such an improved tester valve that does not have a time period during which closure of the tester valve is uncertain or impossible.